The present invention relates to a circuit configuration, or system, for detecting wheel sensor malfunctions intended for automotive vehicle brake systems with electronic anti-lock control and/or traction slip control. Such a circuit configuration includes circuits which process and assess the sensor signals, which ascertain speed, deceleration and acceleration of the individual wheels and compare these values with one another as well as with predefined limit values. In addition, such a circuit configuration includes circuits which disconnect the control at least temporarily once signals or combinations of signals typical of a sensor malfunction are detected.
Monitoring the individual component parts and the operability of an electronically controlled brake system is of great significance. This is because once malfunctions are detected, the conventional functioning of the brakes can be safeguarded by disconnecting the control. Since a great variety of errors and types of errors can occur, the control systems are monitored by different measures in practice.
For instance, one monitoring method involves regularly generating test pulses and observing the reaction of the component parts to these pulses. Another monitoring method involves performing the signal processing in the electronic control unit of the system in parallel in separate circuits and monitoring the output signals generated this way for coincidence. A large number of errors can be detected by the so-called plausibility criteria method. This method of error detection is based on that specific measured values or combinations of signals are not physically possible when the system is intact. Thus, if such values or combinations of signals occur, an error is indicated. However, difficulties are involved in many cases to distinguish between signals caused by malfunctions and correct control signals.
Some types of errors or problems cannot be detected at all, or not reliably or not fast enough by all known monitoring methods. The monitoring of the wheel sensors is among these problems. Erroneous signals occur in a variety of circumstances, e.g. in the presence of too large of an air gap, when the toothed disc loosens or is mounted incorrectly, in the event of pulse generator teeth missing partly or wholly, etc. In these circumstances, it is difficult to determine whether the absence of sensor signals on starting the vehicle is due to a standstill of the wheel or due to the sensor wheel missing. Problems of this kind are known among specialists under the phrase "detection on starting".
An anti-lock system with a safety function is already known from European patent specification EP 0 075 932 B1, wherein specific pairs of signals are detected and an error is identified once a combination of signals occurs which is not possible during fail-safe operation. Specific sensor errors are unidentifiable this way because the signals caused by the malfunction can occur in certain control situations as well during a malfunction.